The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to an electronic ballast with adaptable charge pump power factor correction.
Fluorescent lighting systems are used extensively in industrial facilities and office buildings. Usually, there is more than one lamp in each lighting fixture, and one ballast powers each of those lamps. In a typical large building, the number of lighting fixtures can be in the hundreds or even thousands. Although the amount of power drawn by each ballast is low (e.g., less than 150 watts), the total amount of power consumed by the fluorescent lighting in a single building can reach in the tens of kilowatts. Such a large load can create a negative effect on the AC line, and potentially cause malfunction in sensitive electrical devices such as computers, lab equipment, and medical devices. In order to avoid such effects, there are rather high standards regarding the xe2x80x9cqualityxe2x80x9d of the power (and, thus, the current) drawn by ballasts from the AC line. These standards are embodied in a number of front-end performance requirements, including high power factor (PF), low harmonic distortion (HD), and low line-conducted electromagnetic interference (EMI).
There are three main circuit approaches for providing the desired front-end performance in an electronic ballast. Each has significant shortcomings.
First, there is the xe2x80x9cpassivexe2x80x9d power factor correction (PFC) approach. The circuitry in this approach consists essentially of an iron choke. The choke, which has a high inductive impedance at the AC line frequency (e.g., 60 hertz), typically provides a power factor of greater than 0.95 and a total harmonic distortion of less than 20%. With the addition of xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d capacitors, this approach provides EMI suppression as well. The shortcomings of this approach are high cost, large physical size, and high power dissipation.
A second approach is commonly referred to as xe2x80x9cactivexe2x80x9d power factor correction, which is usually realized by a high frequency boost type converter comprising a MOSFET switch, a small ferrite inductor, and control circuitry for the MOSFET switch. Additionally, a small common-mode ferrite inductor with X and Y capacitors is required for EMI suppression. This approach provides close to unity power factor and a total harmonic distortion of less than 10%. An additional benefit of this approach is that the DC bus voltage (i.e., the voltage provided at the output of the boost converter) remains constant over relatively wide variations in input voltage or load. The shortcomings of this approach include complex circuitry and high material cost.
A third approach is commonly referred to as xe2x80x9ccharge pumpxe2x80x9d power factor correction (PFC), wherein high frequency current from the ballast inverter or output is fed back to the front-end portion of the ballast. In its simplest form, a charge pump circuit consists of a single diode and capacitor; like the two approaches previously described, this approach requires additional circuitry for EMI suppression. Properly designed and implemented, a charge pump circuit can provide front-end performance comparable to that of a boost converter (e.g., close to unity power factor and less than 10% total harmonic distortion), but with considerably less cost, complexity, and physical size.
FIG. 1 schematically illustrates a prior art ballast with a charge pump arrangement. The ballast 20 includes: an EMI filter 40; a full-wave diode bridge 42,44,46,48; a charge pump circuit consisting of inductor 60, capacitor 62, and diode 52; an energy-storage capacitor 58; and a half-bridge inverter 70 that includes two series-connected transistors 72,74 coupled at a junction 76. The ballast is connected to the AC line source 10 via input connections 22,24, and to a fluorescent lamp 12 via output connections 26,32. During operation, the charge-pump circuit works in conjunction with the inverter to increase the power factor of the current drawn from AC line source 10 by injecting an amount of high frequency current from the inverter into the junction between diode bridge 42,44,46,48 and diode 52. This injection of current also acts to boost the DC bus voltage across capacitor 58; the DC bus voltage is dependent on the inverter operating frequency, the capacitance of capacitor 58, and the energy consumed by lamp 12. During steady-state operation, there is a balance between the energy provided by the charge pump (to energy-storage capacitor 58) and the energy consumed by the load (i.e., lamp 12).
A major shortcoming of charge pump circuits lies in the fact that the DC bus voltage is strongly dependent on the load power. More specifically, the DC bus voltage will tend to increase as the load decreases. For example, in the case of removal or failure of lamp 12 (or, in a ballast that power multiple lamps, the removal or failure of even one lamp), the DC bus voltage will jump to an unacceptably high level, which can lead to inverter failure. Thus, ballasts with charge pump circuits necessarily include special protection circuitry for dealing with lamp removal/failure.
Known ballasts with charge pump PFC are intended to work with only one or two lamps connected in series. In the case of lamp removal/failure, a shutdown circuit stops ballast operation. This type of ballast is widely used in the European market, and ballast shutdown in the event of lamp removal/failure is a required feature in Europe.
By contrast, in the North American market, the most widely used ballasts operate anywhere from two to four lamps connected in parallel. Because it is expected that the ballast will continue to operate even if some (but not all) of the lamps fail or are removed, a complete shutdown of the ballast in the event of removal/failure of some of the lamps is not an acceptable option.
What is needed, therefore, is a ballast with charge pump power factor correction that accommodates multiple parallel-connected lamps and that, in the event of removal/failure of some of the lamps, continues to provide power to the remaining lamps without harm to the ballast. A further need exists for a ballast that realizes the aforementioned functionality in an efficient and cost-effective manner. Such a ballast would represent a significant advance over the prior art.